1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly, to a liquid crystal display device and a method for manufacturing the same, in which formation of patterns on a lower substrate is controlled to prevent a seal detachment problem when a conductive seal is used to connect the lower substrate to an upper substrate so as to apply a signal to a common electrode of the upper substrate.
2. Discussion of the Related Art
With growth of an information based society, demands for display devices are gradually increasing. To meet these demands, recently, various kinds of flat panel display devices, such as a Liquid Crystal Display (LCD) device, Plasma Display Panel (PDP), Electro Luminescent Display (ELD), Vacuum Fluorescent Display (VFD), etc., have been studied. Some of these display devices have already been utilized in various equipment.
Of the above mentioned display devices, due to high image quality, low weight, thin thickness, and low power consumption characteristics and other advantages, a liquid crystal display device has been most frequently used as a mobile image display device substituting for a Cathode Ray Tube (CRT). Specifically, various liquid crystal display devices have been developed and used not only as a mobile display device, such as a monitor of a laptop computer, but also as a monitor of a desktop computer and a television having a function of receiving and displaying broadcast signals.
To apply these liquid crystal display devices as a general screen display device into various fields, it is important that a liquid crystal display device can realize a high-resolution, high-brightness and a large-area screen image while maintaining characteristics of low weight, thin thickness, and low power consumption characteristics.
Hereinafter, a conventional liquid crystal display device will be described with reference to the accompanying drawings.
FIG. 1 is a plan view of a conventional liquid crystal display device, FIG. 2 is an enlarged view of the portion A of FIG. 1, and FIG. 3 is a sectional view taken along the line I-I′ of FIG. 2.
As shown in FIG. 1, the conventional liquid crystal display device includes an upper substrate 30, a lower substrate 1 having a larger border area than the upper substrate 30 for formation of a pad portion, and a liquid crystal layer (not shown) filled between the upper substrate 30 and the lower substrate 1.
Each of the upper plate 30 and the lower plate 1 is centrally defined with an active area A/A. A seal pattern 25 is formed around the active area A/A to surround a border of the active area A/A. The seal pattern 25 serves to bond the upper substrate 30 and the lower substrate 1 to each other.
In the above described conventional liquid crystal display device, however, a part of the seal pattern 25 may exhibit deterioration in adhesive strength with respect to an opposite substrate (i.e., the upper substrate or the lower substrate), causing a seal detachment problem wherein the seal pattern peels away from a surface of the opposite substrate. The seal detachment problem may appear as leakage of light at a corresponding region and thus, there is presented an effort to solve this problem.
Now, a reason behind the seal detachment will be described via consideration of the portion A of the conventional liquid crystal display device. FIGS. 2 and 3 illustrate the portion A in enlarged scale.
The lower substrate 1 is formed, on a region thereof corresponding to the seal pattern 25, with a common electrode pattern 20, a gate insulating layer 15 and a protection layer 16 covering the common electrode pattern 20, a contact hole 18 defined by removing predetermined regions of the gate insulating layer 15 and the protection layer 16 to expose the common electrode pattern 20, and a transparent electrode pattern 13 connected to the common electrode pattern 20 through the contact hole 18.
The upper substrate 30 is formed with a black matrix layer 31, an overcoat layer 32, and a common electrode 33.
Here, the common electrode 33 and the transparent electrode pattern 13 are electrically connected to each other via the seal pattern 25 made of a conductive material.
The seal pattern 25 is made of a photo-curable conductive material, and is cured when Ultra-Violet (UV) light is irradiated to the lower substrate 1 or the upper substrate 30. In this case, due to the fact that the black matrix layer 31 is applied to the upper substrate 30 thus acting to prevent light from being irradiated to the seal pattern 25 through the upper substrate 30, UV light is irradiated from the bottom of the lower substrate 1, so as to realize curing of the seal pattern 25.
However, the lower substrate 1 includes the common electrode pattern 20 made of a light shielding metal and extending in a predetermined width or more and in turn, the transparent electrode pattern 13 is formed above the common electrode pattern 20 to have a greater width than the common electrode pattern 20.
Although the transparent electrode pattern 13 is made of a transparent material, the presence of the transparent electrode pattern 13 may reduce light transmissivity by about 18% as compared to when the transparent electrode pattern 13 is absent. Therefore, when UV light is irradiated to the transparent electrode pattern 13, the transparent electrode pattern 13 does not transmit all of the incident light, causing incomplete curing of a sealant. Accordingly, the sealant above the transparent electrode pattern 13 may have a deteriorated photo curing degree and in particular, may be easily affected by extreme environment, more particularly, under conditions of a high-temperature of 60° C. or more, high-humidity, and low-temperature. This disadvantageously increases a possibility of sealant detachment.
In this case, since a part of the seal pattern 25, above the common electrode pattern 20 made of a light shielding metal, has a difficulty in sufficient curing thereof and furthermore, a part of the seal pattern 25, above the transparent electrode pattern 13 around the common electrode pattern 20, has a deteriorated photo curing degree due to a reduction in the transmissivity of UV light, the above described conventional liquid crystal display device has a problem in that the incompletely cured seal pattern 5 may detach from the opposite substrate.
FIG. 4 is a photograph illustrating a seal detachment phenomenon of the conventional liquid crystal display device.
Referring to FIG. 4, there is illustrated a situation in which a seal pattern formed on a lower substrate detaches from an opposite upper substrate. When UV light is irradiated to the lower substrate after a sealant is applied onto a transparent electrode pattern formed on the lower substrate, a common electrode pattern and the transparent electrode pattern do not achieve transmission of a sufficient amount of the UV light, causing incomplete curing of a seal pattern. Therefore, as will be appreciated, the cured sealant has a deterioration in adhesive strength with respect to a common electrode of the upper substrate thereabove, causing formation of a void. With continuous introduction of outside air through the void, there may occur a sealant detachment problem.
Furthermore, as will be appreciated, the seal pattern also has an insufficient adhesive strength with respect to the common electrode pattern of the lower substrate, causing partial seal detachment from the lower substrate. It can be said that this occurs because of low light transmissivity upon photo curing of the sealant as described above. Assuming that the transparent electrode pattern shown in the drawing has a thickness of 40 nm, it was experimentally observed that the light transmissivity drops by about 18% as compared to when the transparent electrode pattern is absent.
The above described conventional liquid crystal display device has the following problems.
In the case of a liquid crystal panel in which a photo-curable conductive seal pattern is used to transmit a signal from a common electrode pattern of a lower substrate to a common electrode of an upper substrate, the greater the width of a transparent electrode pattern below the seal pattern, the lower the light transmissivity. This may cause insufficient abnormal photo curing of a sealant, resulting in a seal detachment problem.
The seal detachment problem may occur even when the common electrode pattern of the lower substrate has a reduced width. Accordingly, it can be appreciated that the curing of the sealant is significantly affected by the presence of the common electrode pattern and the transparent electrode pattern of the lower substrate.